[PATCH v2] ext4: don't report delalloc data as a hole on indirect-mapped inodes
From: Daejun Park
Date: Thu Aug 27 2026 - 00:39:50 EST
When a plain lookup finds no block, ext4_ind_map_blocks() sizes the hole
it reports by counting the empty subtrees under 'partial' in the on-disk
indirect tree. That count knows nothing about delayed allocation, so a
range holding delalloc data that has not been written back yet is
reported as a plain hole. The extent-mapped path does not have this
problem: it trims the hole it found at the first delayed extent before
returning it.
ext4_map_blocks() consults the extent status tree before it calls into
the mapping layer, so a query that starts exactly on the delayed block
still finds it. A query that starts earlier does not, because the hole
reported for the earlier block already spans the delayed one. On a
4k-block filesystem, lseek(SEEK_DATA) from offset 0 on a file whose only
data block is at logical block N and is still dirty returns:
N = 0..11 direct blocks N << 12
N = 12 first single-indirect block N << 12
N = 13..1035 inside that indirect block -1 ENXIO
N = 1036 first double-indirect block N << 12
Blocks 12 and 1036 survive because the query lands on the delayed block
itself. Blocks 13..1035 are swallowed by the 1024-block hole reported
for block 12. fiemap loses the same data for the same reason.
An ext3 filesystem mounted as ext4 hits this during ordinary use: ext3
inodes stay indirect mapped, while mount -t ext4 turns on delayed
allocation even though mounting ext3 with -o delalloc is explicitly
rejected. install(1) from coreutils uses SEEK_DATA to locate data in
its source file, so installing a sparse file that has not been written
back silently produces a destination of the right size holding none of
the data:
truncate -s 1G img && mkfs.ext3 -F img && mount -t ext4 img mnt
echo | dd of=mnt/src bs=1 count=1 seek=64K
install mnt/src mnt/dst
cmp mnt/src mnt/dst # differ: char 65537
Reconciling a hole with the extent status tree does not depend on how
the hole was found, so instead of open coding it a second time, split
the delalloc handling out of ext4_ext_determine_insert_hole() into
ext4_determine_insert_hole(), which takes the hole the caller located,
and call that from ext4_ind_map_blocks() as well. All the special cases
and the comments explaining them stay in one place.
Two things change for indirect-mapped inodes. They now cache the holes
they report, as the extent-mapped path already does: ext4_punch_hole()
already puts EXTENT_STATUS_HOLE on such inodes, ext4_map_blocks() and
ext4_da_map_blocks() already handle a cached hole, the latter by
replacing it with the delayed extent, and consecutive hole entries
merge, so the tree does not grow with the length of a scan. And when a
delayed extent covers the queried block itself - a race, since
ext4_map_blocks() found nothing in the tree just before - the hole is
now reported only up to the end of that extent instead of in full.
The indirect path passes hole_start == lblk, so the "delalloc extent in
front of the queried range" case cannot be reached there:
__es_tree_search() never returns an extent ending before the block it
was searched for.
With this the sweep above returns N << 12 for every N, while the
nodelalloc and extent-mapped controls are unchanged. generic/225,
generic/285, generic/286, generic/436, generic/448 and generic/490 pass
on ext4 made both with and without the extent feature.
Fixes: facab4d9711e ("ext4: return hole from ext4_map_blocks()")
Reported-by: Alexander Monakov <amonakov@xxxxxxxxx>
Closes: https://lore.kernel.org/linux-ext4/594c17d9-c00f-e485-96fb-cedf27ce3aa3@xxxxxxxxx/
Suggested-by: Jan Kara <jack@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Daejun Park <daejun7.park@xxxxxxxxxxx>
---
v1: https://lore.kernel.org/linux-ext4/20260826052446epcms2p5a415be5fbfe23b8a32786f0ae6d05aea@epcms2p5/
v2: rather than open coding the delalloc trim in ext4_ind_map_blocks(),
split it out of ext4_ext_determine_insert_hole() into
ext4_determine_insert_hole() and call that from both paths, as Jan
suggested. Two things follow from sharing the whole function, both
noted in the commit message: indirect-mapped inodes now cache the
holes they report, and a delalloc extent covering the queried block
now shortens the reported hole instead of leaving it whole.
fs/ext4/ext4.h | 3 +++
fs/ext4/extents.c | 26 +++++++++++++-------------
fs/ext4/indirect.c | 11 +++++++++++
3 files changed, 27 insertions(+), 13 deletions(-)
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 7fd078de265d..f5eff74631a4 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -3893,6 +3893,9 @@ extern void ext4_ext_tree_init(handle_t *handle, struct inode *inode);
extern int ext4_ext_index_trans_blocks(struct inode *inode, int extents);
extern int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
struct ext4_map_blocks *map, int flags);
+ext4_lblk_t ext4_determine_insert_hole(struct inode *inode, ext4_lblk_t lblk,
+ ext4_lblk_t hole_start,
+ ext4_lblk_t hole_len);
extern int ext4_ext_truncate(handle_t *, struct inode *);
extern int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start,
ext4_lblk_t end);
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 836396ea7912..e2b7565e2cb4 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4190,21 +4190,19 @@ static int get_implied_cluster_alloc(struct super_block *sb,
}
/*
- * Determine hole length around the given logical block, first try to
- * locate and expand the hole from the given @path, and then adjust it
- * if it's partially or completely converted to delayed extents, insert
- * it into the extent cache tree if it's indeed a hole, finally return
- * the length of the determined extent.
+ * Adjust the hole [@hole_start, @hole_start + @hole_len) the caller found
+ * around the queried block @lblk if it's partially or completely converted
+ * to delayed extents, insert it into the extent cache tree if it's indeed a
+ * hole, finally return the length of the determined extent starting at
+ * @lblk. @hole_start must not be behind @lblk.
*/
-static ext4_lblk_t ext4_ext_determine_insert_hole(struct inode *inode,
- struct ext4_ext_path *path,
- ext4_lblk_t lblk)
+ext4_lblk_t ext4_determine_insert_hole(struct inode *inode, ext4_lblk_t lblk,
+ ext4_lblk_t hole_start,
+ ext4_lblk_t hole_len)
{
- ext4_lblk_t hole_start, len;
+ ext4_lblk_t len = hole_len;
struct extent_status es;
- hole_start = lblk;
- len = ext4_ext_find_hole(inode, path, &hole_start);
again:
ext4_es_find_extent_range(inode, &ext4_es_is_delayed, hole_start,
hole_start + len - 1, &es);
@@ -4371,9 +4369,11 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
* we couldn't try to create block if flags doesn't contain EXT4_GET_BLOCKS_CREATE
*/
if ((flags & EXT4_GET_BLOCKS_CREATE) == 0) {
- ext4_lblk_t len;
+ ext4_lblk_t hole_start = map->m_lblk, len;
- len = ext4_ext_determine_insert_hole(inode, path, map->m_lblk);
+ len = ext4_ext_find_hole(inode, path, &hole_start);
+ len = ext4_determine_insert_hole(inode, map->m_lblk,
+ hole_start, len);
map->m_pblk = 0;
map->m_len = min_t(unsigned int, map->m_len, len);
diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
index 5aec759eed70..9dc0759c88ef 100644
--- a/fs/ext4/indirect.c
+++ b/fs/ext4/indirect.c
@@ -586,6 +586,17 @@ int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
for (i = partial - chain + 1; i < depth; i++)
count = count * epb + (epb - offsets[i] - 1);
count++;
+
+ /*
+ * The count knows nothing about delayed allocation, so let
+ * the common helper reconcile it with the extent status
+ * tree. Clamp it first: with a large block size a subtree
+ * can be bigger than the logical block space.
+ */
+ count = umin(count, EXT_MAX_BLOCKS - map->m_lblk);
+ count = ext4_determine_insert_hole(inode, map->m_lblk,
+ map->m_lblk, count);
+
/* Fill in size of a hole we found */
map->m_pblk = 0;
map->m_len = umin(map->m_len, count);
base-commit: 9091c97be34083587a75db174aab51551d8e8543
--
2.43.0